Good luck on the rescue effort.

If you do end up continuing to feed him, maybe the intolerant cat will come around eventually. In my experience they usually do, even if it's at best a live and let live relationship.

Dog relationships, because we have a reasonable knowledge of pack behaviour, are so much easier to figure out. Domestic cats seem able to choose among being singletons, living and hunting alone; duos, which you often see where a pair of littermates sticks together, sleeps together, etc.; and colonies, where a multitude of cats manage to develop pecking orders, group 'rules', matriarchal units, and so on - very complicated and fascinating behaviours.